We return now to the mystery of a doctor married six times and at least two of his wives met a tragic end instead of a happily ever after. Eight months after we first went looking for answer, you will never guess why he is finally under arrest tonight. Here again is ABC's Jim Avila. Three people go to sleep in this house on a winter night in Alabama. Only two wake up. The husband, Dr. Mike, says maybe his wife fell down the stairs a few days earlier. But no one, not him, nor her mother who was there, saw or for that matter heard Shirley die. What happened? No better word, slammed my sister's head against a floor or something hard, fractured the bone in the back of her bed. Dislocated her brain from the front of her head. And she died from the results of it. Reporter: So is that possible? The toxicology report found no poison or fatal doses of drugs. And even as it makes clear the cause of death, the autopsy fails to answer the really big question -- the manner of death, how Shirley got that fatal injury president? Medical examiner writes undetermined. Accident, homicide, can't tell. But there was an investigation by police and the Alabama bureau of investigation. It went on for a year. It resulted in a 184-page report to the prosecutor. I believe, I will go to my grave believing, that sometime between midnight and 7:00, he went in the bedroom and took her life. We tried to make an appointment to see the doctor to ask him about his terrible luck, how and why two wives had died early, sudden deaths and two others suspect he tried to poison them. But the doctor wouldn't see us. Turned down our request for an interview. So we want to see him. We found him living in Pensacola, Florida, and not alone. What you are doing? The woman with him and threatening to sue as I beck pedal was known as Lou Ann. She had known Dr. Meike when Shirley was still alive. The question is just how well? I do believe he was having an affair on mother. The woman that he is now married to, I believe is the woman that he was seeing prior to mother's death. Reporter: Married to, that's right, Lou Ann now goes by the name of Maggie. She is wife number six. Are you currently in a relationship with anybody else now? I go out with people just to -- to be with them. Reporter: Her name had come up in Dr. Mike's interview three months after Shirley's death when police asked about his love life. I have been to the movies with Louann and I have actually babysat Lou Ann's two sons, not really dating, dating. I mean, I have, I see. None of them are dates per Se. No. No. Dr. Mike denied the police that he cheated on Shirley. But Sharon says she believes it could be another motive for wanting Shirley out of the way. There is too many red flags with what I have discovered in reference to her. When we found Dr. Mike last year in a fast food parking lot, wife number six, was not happy to see us. But we did get to ask that critical question. That's all we wanted. All we wanted. Did you murder Shirley? No, I did not. When Shirley met this man she had half a million in liquid assets. A home and 15 acres of land. What happened to your sister when the money ran out? She died. He told us back then he had no money was living on food stamps and couldn't afford a lawyer. But he need one now. Just three days ago, police arrested the doctor for the murder of Shirley four years after her death, eight months after ABC news first broadcast the story that the doctor is finally behind bars. The family's private investigator told us today, that without the original "20/20" report, he doubts that might ever have happened. If convicted, he faces a death sentence or life in prison. His court appointed attorney told ABC news, that he will be acquitted for reasons it took the state of Alabama to indiet my client and obvious that those reasons are not favorable to the prosecution. The family says they're ecstatic but are staying mute for now to allow law enforcement to do its job. But any time he need to remember, little brother Ricky, drives up to that quiet spot in highrock and listens for the sound he has been missing. His big sister's voice. ? ? ? amazing grace ? ? how sweet the sound ? And some times just sit there and listen to her sing, amazing grace. ?

This transcript has been automatically generated and may not be 100% accurate.

Alabama's Woman's Mysterious Death Raises Suspicions

Part 1: Four years into her marriage to Michael Wohlschlaeger, Shirley Seitz became very ill.

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